Spark plugs used in internal-combustion engines include an insulator formed of, for example, an alumina-based sintered body containing alumina as a main component. The alumina-based sintered body is typically formed by sintering a powder mixture including a sintering additive including, for example, SiO2 (e.g., International Publication No. 2013/128525). The technique disclosed in International Publication No. 2013/128525 improves dielectric strength in a high-temperature environment by controlling the grain size of a crystal phase.
Regarding the above-mentioned technique, there has been a demand for further improvement in terms of the dielectric strength in a high-temperature environment.
The present invention is made to meet the foregoing demand. An advantage of the present invention is a spark plug having an insulator that improves dielectric strength in a high-temperature environment.